1. Field of the Invention
The present invention relates to a transmission circuit for use in a communication apparatus for mobile telephony, wireless LAN, or the like. More particularly, the present invention relates to a transmission circuit which outputs a transmission signal having high linearity independently of the bandwidth and operates with high efficiency, and a communication apparatus employing the transmission circuit.
2. Description of the Background Art
There is a demand for a communication apparatus for mobile telephony, wireless LAN, or the like which can secure the linearity of an output signal and operate with low power consumption even when it operates within a broad bandwidth. In such a communication apparatus, a transmission circuit is employed which outputs a transmission signal having high linearity independently of the bandwidth and operates with high efficiency. Hereinafter, conventional transmission circuits will be described.
As a conventional transmission circuit, for example, there is a transmission circuit which utilizes a modulation method, such as quadrature modulation or the like, to generate a transmission signal (hereinafter referred to as a quadrature modulation circuit). Note that the quadrature modulation circuit is widely known and will not be described. As a conventional transmission circuit which has a smaller size and a higher efficiency than those of the quadrature modulation circuit, for example, there is a transmission circuit 500 illustrated in FIG. 24. FIG. 24 is a block diagram illustrating a configuration of the conventional transmission circuit 500. In FIG. 24, the conventional transmission circuit 500 comprises a signal generating section 501, an angle modulating section 502, a regulator 503, an amplitude modulating section 504, an output terminal 505, and a predistortion compensating section 506.
In the conventional transmission circuit 500, the signal generating section 501 generates an amplitude signal and a phase signal. The amplitude signal and the phase signal are input to the predistortion compensating section 506. The predistortion compensating section 506 distorts the input amplitude signal and phase signal so as to compensate for the nonlinearity of the amplitude modulating section 504. The amplitude signal output from the predistortion compensating section 506 is input to the regulator 503. The regulator 503 supplies a voltage depending on the input amplitude signal to the amplitude modulating section 504.
The phase signal output from the predistortion compensating section 506 is input to the angle modulating section 502. The angle modulating section 502 subjects the input phase signal to angle modulation to output an angle-modulated signal. The angle-modulated signal output from the angle modulating section 502 is input to the amplitude modulating section 504. The amplitude modulating section 504 subjects the angle-modulated signal to amplitude modulation using the voltage supplied from the regulator 503, to output an angle-modulated and amplitude-modulated signal. This modulated signal is output as a transmission signal from the output terminal 505. Thus, the conventional transmission circuit 500 compensates for the nonlinearity of the amplitude modulating section 504 using the predistortion compensating section 506, thereby outputting a transmission signal having high linearity.
However, the conventional transmission circuit 500 does not take into consideration a change in characteristics due to temperature of the amplitude modulating section 504. Therefore, when the characteristics of the amplitude modulating section 504 change due to the temperature, the linearity of the transmission signal is deteriorated.
U.S. Pat. No. 6,295,442 (hereinafter referred to as Patent Document 1) discloses a transmission circuit which compensates for distortions of an amplitude signal and a phase signal, depending on a change in characteristics of an amplitude modulator. FIG. 25 is a block diagram illustrating a configuration of a conventional transmission circuit 600 disclosed in Patent Document 1. In FIG. 25, the conventional transmission circuit 600 comprises a summer 603, a phase compensation table 604, an amplitude compensation table 605, a comparator 606, a phase modulator 607, an amplitude modulator 608, an amplitude detector 609, a comparator 610, a mixer 611, and an amplitude limiter 612.
A phase signal 601 is compensated for by the summer 603, depending on a set value in the phase compensation table 604, and thereafter, is input to the phase modulator 607. The phase modulator 607 subjects the input phase signal to phase modulation to generate a phase-modulated signal. The phase-modulated signal generated by the phase modulator 607 is input to the amplitude modulator 608. An amplitude signal 602 is compensated for, depending on a set value in the amplitude compensation table 605, and thereafter, is input to the amplitude modulator 608. The amplitude modulator 608 subjects the phase-modulated signal input from the phase modulator 607 to amplitude modulation using the amplitude signal input via the amplitude compensation table 605. The signal amplitude-modulated by the amplitude modulator 608 is output as a transmission signal.
The transmission signal is input to the amplitude detector 609. The amplitude detector 609 detects an amplitude component included in the transmission signal, and outputs the detected amplitude component to the comparator 606. The comparator 606 compares the amplitude component included in the transmission signal with the amplitude signal 602, and depending on the result of the comparison, updates the set value of the amplitude compensation table 605.
The transmission signal is also input to the amplitude limiter 612. The amplitude limiter 612 limits the amplitude component included in the transmission signal to output only a phase component included in the transmission signal. The phase component included in the transmission signal is multiplied by the phase-modulated signal in the mixer 611, and the result is input to the comparator 610. The comparator 610 compares the phase component multiplied in the mixer 611 with a set value of the phase compensation table 604, and depending on the result of the comparison, updates the set value of the phase compensation table 604.
Thus, the conventional transmission circuit 600 updates set values of the phase compensation table 604 and the amplitude compensation table 605, depending on a phase component and an amplitude component included in a transmission signal output from the amplitude modulator 608. Therefore, even if characteristics of the amplitude modulator 608 are changed due to temperature or the like, a transmission signal having high linearity can be generated.
However, in the conventional transmission circuit 600, set values of the phase compensation table 604 and the amplitude compensation table 605 are updated at any time, depending on a phase component and an amplitude component included in a transmission signal output from the amplitude modulator 608. Therefore, a complicated feedback control is required, so that a number of parts (e.g., the comparator 606, the amplitude detector 609, the comparator 610, the mixer 611, the amplitude limiter 612, etc.) need to be provided. Therefore, the conventional transmission circuit 600 has a large circuit scale. In addition, in the conventional transmission circuit 600, loss occurs when the transmission signal output from the amplitude modulator 608 is split into a plurality of signals, resulting in large power consumption for a transmission circuit.